Please review my 6AQ5 Amp

[PRNDL]

I've been modifying an old Dynavox 6AQ5 amp for guitar and thought it might be a good idea to have someone else review my schematics.

I'm waiting for parts to the tone stack. The amp sounds pretty good without it, and seems to have enough volume. It distorts fairly early (around 3:00), but adding the tone stack will change that.

I used a simple concertina phase inverter without feedback in order to increase gain. It was an experiment that worked great.

There is an empty spot for a second preamp tube, but I decided not to use it in favor of the simpler design.

Thanks!!

Schematics
http://www.naturdoctor.com/Downloads/Dynavox_1.pdf

[Tom Bavis]

As drawn, you have unequal drive to the output tubes - it's close to being single-ended! The plate and cathode load resistors must be equal for equal drive levels (47K or so?). This will require two additional changes: the grid of the concertina will be biased positive, so it will need a coupling cap, and gain will be greatly reduced (to a gain of ONE), so you'll need the extra preamp stage. If you have a seven pin socket, use a 6AV6 or 6AQ6 (half of a 12AX7), or even a 6AU6. If it's nine pin, you have more choices.

Also - R24 CAN'T be 22K - typo?

[PRNDL]

Thanks Tom ... I'm glad I posted!

Yes, R24 is most likely not 22K. I haven't measured the resistors in the power supply.

It looks like I'll have to order some more parts for the extra tube/gain stage.

Is there a benefit for switching from cathodyny to to a long-tailed phase invertor?
I know that you get gain with the latter, but is there a big difference in tone/distortion?

[kevinkr]

LTP is generally the standard driver topology in pp guitar amplifiers, I'd give it a try.. I think with the LTP you might have almost "enough" gain without an additional stage, provided you don't plan on adding NFB later on.

[PRNDL]

I've updated the amp by adding another 12AX7 and built it, but it doesn't sound right. It sounds OK with very low volume.
Either there's a mistake somewhere, or some of the caps I had on hand were the wrong values.
I'm concerned about the coupling caps (C1=.02, C5=.01) and the phase invertor cap (C6=.1, which should be .01). My guess is that C6 is off.

I've had it for the evening, and am going to play guitar.

Any suggestions?

Thanks again!

[Brian Beck]

You're feeding the grid of V2 through a 1M resistor. Try connecting the right end of C5 to the grid.

[kevinkr]

C5 should go to pin 2 of the driver ltp.. Hopefully not wired as drawn..
Will think about this over the wk end.

[PRNDL]

Would you believe that it sounded better with C5 wired incorrectly?

Oh well.

Here are some voltages

V1.a 6=158, 8=1
V1.b 1=129, 3=1

V2: 1=173, 2=18, 3=51; 6=174, 7=18, 8=51

V3: 5=261, 6=237, 2=15
V4: 5=260, 6=238, 2=15

PS: A=287, B=265, C=237

The amp squeals with the volume off zero, so I may need to add NFB to reduce overall gain. I'm also driving the 6AQ5's slightly higher than the max (250).

[PRNDL]

I've read several times that the 6AQ5 is a similar to a 6V6, so I decided to look in the Book of Fender and found that the 6G3 Deluxe has 6V6 with LTP PI.

Attached is a revised circuit.

[Tom Bavis]

You have the first three stages and the output screens all connected to the same power supply tap. You might try adding a 10K resistor and another electrolytic to supply the two input stages. This "de-coupling" may solve the squealing...